This invention relates to chemical reactors for the conversion of a reaction fluid while indirectly heating a reaction with a heat exchange fluid.
In many industries, like the petrochemical and chemical industries for instance, the processes employ reactors in which chemical reactions are effected in the components of one or more reaction fluids by contact with a catalyst under given temperature and pressure conditions. Most of these reactions generate or absorb heat to various extents and are, therefore, exothermic or endothermic. The heating or chilling effects associated with exothermic or endothermic reactions can positively or negatively affect the operation of the reaction zone. The negative effects can include among other things poor product production, deactivation of the catalyst, production of unwanted by-products, and in extreme cases, damage to the reaction vessel and associated piping. More typically, the undesired effects associated with temperature changes will reduce the selectivity or yield of products from the reaction zone.
One solution to the problem has been the indirect heating of reactants and/or catalysts within a reaction zone with a heating or cooling medium. The most well known catalytic reactors of this type are tubular arrangements that have fixed or moving catalyst beds. The geometry of tubular reactors poses layout constraints that require large reactors or limit throughput.
Indirect heat exchange has also been accomplished using thin plates to define alternate channels that retain catalyst and reactants in one set of channels and a heat transfer fluid in alternate channels for indirectly heating or cooling the reactants and catalysts. Heat exchange plates in these indirect heat exchange reactors can be flat or curved and may have surface variations such as corrugations to increase heat transfer between the heat transfer fluids and the reactants and catalysts. Although the thin heat transfer plates can, to some extent, compensate for the changes in temperature induced by the heat of reaction, not all indirect heat transfer arrangements are able to offer the complete temperature control that would benefit many processes by maintaining a desired temperature profile through a reaction zone. Many hydrocarbon conversion processes will operate more advantageously by maintaining a temperature profile that differs from that created by the heat of reaction. In many reactions, the most beneficial temperature profile will be obtained by substantially isothermal conditions. In some cases, a temperature profile directionally opposite to the temperature changes associated with the heat of reaction will provide the most beneficial conditions. An example of such a case is in dehydrogenation reactions wherein the selectivity and conversion of the endothermic process is improved by having a rising temperature profile or reverse temperature gradient through the reaction zone. A specific arrangement for heat transfer and reactant channels that offers more complete control can be found in U.S. Pat. No. 5,525,311, the contents of which are hereby incorporated by reference.
Heating reactants within a reaction zone poses a number of limitations on the reactor arrangement and the operation of the process. A heat exchange reactor typically needs to operate with a large fluid mass flow rate of the heat transfer fluid in order to provide adequate mass flux of the heat transfer fluid over the heat transfer surfaces. Failure to maintain the adequate heat transfer fluid mass flux across the heat transfer surfaces will result in inadequate heating and a loss of benefit from providing the internal heating within the reaction zone. The heat exchange reaction section may be divided into multiple heat exchange reactor sections. Nevertheless each heat exchange reactor section still requires a high mass flux rate to provide adequate heating across all of the heat transfer surfaces.
Even with separate reaction zones or reaction stacks, as they are referred to herein, the maximum temperature for the heat transfer fluid also remains limited. Constraints on the temperature of the heat transfer fluid as providing heating to the reactants can typically apply to minimum or maximum values. Minimum allowable temperature must be high enough to induce a reaction rate that exceeds what would ordinarily be obtained from an adiabatic process. However, at the same time, the maximum temperature at which the heating fluid enters the heat transfer zone must not heat the reactants to a temperature that can cause a lack of selectivity in the products produced, or worse, a decomposition of the products already produced.
For example, in the dehydrogenation of ethylbenzene, the process requires that the endothermic heat of reaction be supplied internally or externally. In an adiabatic reactor operation, the sensible heat in the feed stream provides the endothermic heat of reaction. Mixing a large quantity of super heated steam to the ethylbenzene feed increases the available sensible heat. Limitations in the ability to provide heating by sensible heat restricts the maximum allowable temperature drop across the reactor. However, using excessive steam temperatures to maintain a minimum reaction temperature will exceed the maximum sensitivity temperature for the ethylbenzene and begin its decomposition. Furthermore, designing equipment for the more severe operating conditions significantly increases its cost. Again, the endothermic heat of reaction may also be supplied by indirect heat exchange from an appropriate heat transfer fluid. Nevertheless, providing sufficient mass flux to all of the heat transfer surfaces will require a high heat transfer fluid mass flow rate which would lead to larger equipment sizes and higher processing costs.
Accordingly, it is an object of this invention to reduce the heat transfer fluid mass flow rate required to provide the necessary heat transfer fluid mass flux to maintain a high reaction temperature without exceeding the sensitivity temperature of the reactants or the products produced by their reaction.
It is a further object of this invention to provide a reactor apparatus for the indirect heating of a reactant stream in a reaction zone while conserving heat and reducing the necessary mass flow rate to provide a given mass flux over the heat transfer surfaces.
This invention uses a multiple-pass heat exchange configuration to heat reactants indirectly with a heat exchange fluid in groups of stacked plates that provide reaction zones having reaction channels and heat exchange channels. The multiple pass arrangement of this invention significantly improves heat transfer. In turn the multiple-pass heat exchanger effectively reduces the heat transfer fluid mass flow rate for endothermic reacting systems while maintaining the equivalent overall heat flux across the heat transfer surfaces that separate the reaction channels from the heating channels. This arrangement reduces the size of the heat transfer equipment. The invention may also be used to increase the heat flux while maintaining a constant heat transfer fluid mass flow rate. The invention uses a series flow of the heat transfer fluid through reactors that receive a parallel flow of reactants. This combination of flows permits the control of the process site temperature profiled in the reactant channels to a variety of desired shapes. The process also enhances the isothermal operation of a reaction zone by more uniformly distributing heat along the catalyst beds. The arrangement improves process and heat transfer flow distributions for co-current, counter-current, or cross-current operations. Serial flow maintains the heat transfer fluid mass flow rates across the heating channels for multiple reaction stacks and reduces capital and operating costs thereby lowering equipment sizes and utility requirements. The flexibility for controlling the temperature of the heat transfer fluid facilitates the use of a variety of heat transfer fluids such as steam, flue gas, liquid sodium, molten salt, and in-situ combustion to be used more efficiently.
This invention can be particularly useful with the use of high heat capacity heating fluids such as molten salts and liquid metals. In particular, liquid sodium has a heat capacity and thermal conductivity that are, on a volumetric basis, superior to most heat transfer mediums. Liquid sodium is known to work for use in dehydrogenation processes including paraffins and aromatics. Despite its excellent heat transfer properties even an excellent heating medium such as liquid sodium may require a mass ratio of heating fluid to process feed as high as 30 to obtain isothermal profiles. The heat transfer piping circulates liquid sodium or other heating fluid the multiple reaction stacks in series flow while the process piping passes the feed through the reaction stacks in parallel. In this manner, for each reaction stack or individual heat transfer reactor provided: in parallel, the mass flow rate of the process feed is reduced inversely to the total number of parallel reaction zones while the total of the heating fluid mass flux to heat transfer surfaces is maintained through the reheating of the heating fluid between the series of reactors. The heating fluid of this invention is not limited to liquid metal or molten salts and may include lower heat capacity fluids such as, hot oil, steam, or flue gas. The reheating of flue gas may be improved by the addition of small amounts of methane or other flue for direct combustion in the heat transfer medium.
The invention may be particularly useful for providing a heating fluid by catalytic combustion of a fuel stream. The rate of combustion of the fuel mixture could be controlled thereby distributing the heat release across more of the heat transfer surface to improve the process reaction profile in terms of selectivity and activity in the reaction channels.
This invention has been found to be particularly effective in the dehydrogenation of ethylbenzene to produce styrene. By improving the uniformity of the heat transfer rate across a heat transfer surface, this invention increases the selectivity for styrene production by up to 1.2% over traditional ethylbenzene dehydrogenation processes. Where steam was used as the heating fluid, the invention improved the usual production over that achieved by adiabatic reactors through the use of multiple-stage heating fluid flow through a plurality of parallel ethylbenzene dehydrogenation reactors. In particular, this invention improves the use of steam as a heating medium in the ethylbenzene dehydrogenation process. Processes for the production of styrene typically require a large amount of steam. This invention uses the steam first as the heating fluid and then injects the steam after heat transfer into the process feed to satisfy the required steam-to-oil ratio. Low steam-to-oil ratios are preferred to reduce utilities and operating costs. This invention facilitates the use of low steam mass flow rates while still providing the necessary heating into the reaction channels. Metallurgical limitations of the reactor will generally restrict steam temperatures to below 800xc2x0 C. and, more practically, to below 650xc2x0 C. This invention increases the heat flux for the steam by piping the steam in series to the heating channels of the reaction stacks and by reheating the steam up to 16 times.
Accordingly, in one embodiment, this invention is a process for contacting reactants with a particulate catalyst in a channel reactor while indirectly contacting the reactants with a heating fluid. The process retains catalyst particles in a plurality of reaction stacks. Each reaction stack contains a plurality of vertically and horizontally extended reaction channels in a plurality of vertically and horizontally extended heating channels for providing indirect heat exchange. A reactant stream passes to at least two of the reaction stacks in parallel flow and contacts the catalyst therein. A heating fluid passes through the heat exchange channels of at least two reaction stacks to create series flow of heating fluid through the reaction stacks. The heating fluid undergoes reheating as it passes from one reaction stack to another reaction stack. The heating fluid is recovered from the last reaction stack in the series of reaction stacks. At least a portion of the heating fluid returns to the first reaction stack in the series. The process recovers a reactant stream from the plurality of reaction stacks.
In another embodiment, this invention is a channel reactor apparatus for contacting reactants with a particulate catalyst, indirectly heating the reactants with a heating fluid and reheating the heating fluid with a heat source in a heater. The apparatus contains a plurality of reaction stacks with each reaction stack comprising a plurality of parallel plates extending vertically and horizontally. The reaction stacks define heating channels and reaction channels. A distribution system passes a reactant stream to the reaction channels of each reaction stack in parallel flow. A withdrawal system collects the parallel flows of reacted reactants from each reaction stack. The heating fluid delivery conduit delivers a heating fluid to the heating channels of one of the reaction stacks located in a lead position. A plurality of intermediate conduits pass the heating fluid in series flow from the reaction stack in the lead position through the heating channels of reaction stacks located in the intermediate position and finally to a reaction stack located in an end position. A heating fluid recovery conduit recovers the heating fluid from the reaction stack in the end position. A heater reheats the heating fluid that passes through each intermediate conduit.
Additional embodiments, arrangements, and details of this invention are disclosed in the following detailed description of the invention.